Magnetic resonance imaging (MRI) is a state of the art imaging technology which allows cross sectional viewing of objects like the human body with unprecedented tissue contrast. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules. The basis of both NMR and MRI is the fact, that atomic nuclei with none zero spin have a magnetic moment. In medical imaging, usually nuclear hydrogen atoms are studied since they are present in the body in high concentrations for example water. The nuclear spin of elementary particles can resonate at a resonance frequency, if a strong DC magnetic field (B0 field) is applied. The magnetic resonance (MR) frequency is determined by the level of the magnetic flux. In an MRI scanner, the magnetic field matches a selected resonance frequency only at one position in space. By varying this position step by step, an image can be measured.
The needed strong DC magnetic field is typically generated by superconducting magnets. In order to vary these fields, such that it matches a given radio frequency only at one position, a field gradient is generated using gradient coils. Thereby, the field gradient can vary over time to achieve a scan. The frequency range in the gradient coils is low and reaches up to a maximum of 10 kHz.
Typically, in MRI apparatus the gradient coils are connected to respective gradient amplifiers. The gradient coils are driven by electrical currents of several hundreds of Amperes, which need to be accurately controlled in the range of mA in order to assure an acquisition of MRI images at high quality and precision.
This requires an accurate control of the gradient amplifier output which can be for example performed by control circuits using feedback loops.
For example U.S. Pat. No. 6,285,304 B1 discloses an analogue-to-digital converter circuit and control device for a gradient amplifier of a magnetic resonance imaging system.
The major disadvantage with such kind of control devices for gradient amplifiers is that these control devices require high precision electronics, like high precision digital-to-analogue converters. Thus, the requirements of high precision, resolution and stability makes it impossible to use commercial analogue-to-digital converters or digital-to-analogue converters to provide a full digital controlled gradient amplifier for MRI applications.